1. Field of the Invention
The present invention pertains to systems for sensing the angular position of a rotary shaft or other member. In particular, the invention pertains to a system associated with such a rotary member and comprising an operator for producing a programmed response in accordance with the angular position of the rotary member. The invention finds application in many fields including that of rotary valves. It may be used to indicate the position of the rotary valve stem or the coaxial shaft of an actuator for the valve and thus the position of the attached valve element whether it be open, closed, or in some intermediate position. It can also be used to produce some other programmed response to the valve stem, actuator shaft and/or valve element position. For example, it may be associated with the valve actuator so that the speed of rotation of the valve is controlled as a function of the position of the valve stem, actuator shaft and/or valve element.
2. Brief Description of the Prior Art
One of the most common approaches in such systems in the past has been to connect a potentiometer to the rotary member whose angular position is to be sensed. The potentiometer was, in some cases, directly mechanically connected to the rotary member and, in other cases, indirectly mechanically connected to the rotary member via a linkage of gears or the like. In either case, the potentiometer would be driven by the rotary member and would produce an output voltage proportional to the position of the rotary member.
This approach has numerous disadvantages. One of these is that the various parts, being mechanically connected to one another, are subject to wear and backlash leading to inaccuracy in the position sensing particularly as the system ages. The windings of the potentiometer are also subject to wear and this can cause complete failure of the sensing apparatus. Furthermore, such systems are quite expensive due to the mechanical complexity and to the need to use high quality parts in an effort to combat wear and backlash problems.
U.S. Pat. No. 3,828,188 to Matula briefly describes a prior art system in which a mirror rotates with a tension arm and reflects greater or lesser amounts of light from an external light source. This system appears to lack the precision and versatility required in many of the applications of the present invention. Furthermore, the system is adapted for relatively light duty rotary machinery such as tape decks and would not be suitable for use with apparatus such as valves and valve actuators which are subjected to much more abuse.
U.S. Pat. Nos. 3,767,992 and 3,770,965 to Edwards et al. disclose somewhat more sophisticated systems in which a control device limits the amount of light either reflected from or transmitted across the control device. These systems also lack the precision and versatility required in many applications. In particular, they lack means for limiting the light reflected from or transmitted across the control device to a relatively small area so that small increments of movement of the control device can be detected. Additionally, the Edwards systems fail to provide adequate isolation of the light detection member from light sources other than the control device. Neither the Matula nor the Edwards et al patents disclose any way of applying their respective systems to rotary valves and actuators, one of the primary environments of interest in the present invention.
Still another system is briefly disclosed in Machine Design, Feb. 6, 1975. This system is based on light transmission rather than light reflection. Thus it uses considerably more space than is desirable and requires a separate member for the control device rather than allowing it to be incorporated on or in a rotary part already present in the rotary apparatus. Furthermore, the system lacks versatility in that the configuration of the light transmission limiting area is not readily altered.
All of the above-described prior art systems have disadvantages in terms of undue weight and expense.